Fluorescence Quenching and Highly Selective Adsorption of Ag⁺ Using N-Doped Graphene Quantum Dots/Poly(vinyl alcohol) Composite Membrane

Fabrication of composite adsorbents with exceptional fluorescence performance has attracted increasing attention. In this study, a nitrogen-doped graphene quantum dots (NGQDs)/poly(vinyl alcohol) (PVA) composite membrane (NGQDs-PVA) with high adsorption selectivity and fluorescence quenching for Ag+ was prepared via simple cross-linking. The SEM characterization showed that the NGQDs were uniformly distributed in PVA, indicating that the material exhibited fluorescence characteristics of quantum dots. C–O, −COOH, −NH2, and −OH functional groups in NGQDs-PVA complexed with a great amount of Ag+, and −NH– reduced a small amount of Ag+ to Ag0. The pseudo-first-order kinetic and Langmuir models were used to describe the adsorption process of heavy metals using the composite. The maximum adsorption capacity was 317.35 mg/g (Langmuir model fitting, pH = 4, T = 40 °C). When the fluorescence of NGQDs-PVA was quenched as the number of Ag+ adsorbed increased, the change in fluorescence intensity was used to qualitatively quantify the adsorption process.